Questions about print resolution are among the questions we are regularly asked:
Indeed, what could be more important for a photographer than making sure the print quality will live up to expectations? The question is simple, the answer a little less so. We are going to try to debunk a number of myths and more or less questionable rules of thumb.
The definition of resolution is sometimes misunderstood. It is quite simply the number of dots per unit of length.
300 dpi = 300 dots per inch = 300 dots in 2.54 cm
Printers very often print at a different resolution in the direction of the raster (the direction of the print headprinting) and in the grain direction (the direction of paper travel).
This is the case with the latest-generation printers we use. They print at a resolution of 2880 x 1440 dpi. That means more than 4 million (1440 x 2880) micro-droplets of ink can be printed on a surface of 1 square inch (6.45 cm²)! On a square measuring 1 cm per side, that means more than 640,000 droplets of just a few picoliters can be projected!
If we add that each of the several billion droplets forming a 60 x 90 art print can be chosen from 10 different colors, that its volume (and therefore its size) can also be adjusted (3.5 picoliters or more), and finally that these printers print at a speed of 6.45 cm² per second at this resolution, it is easy to imagine the feat these machines accomplish.
Does that mean the images to be printed must be supplied at that same resolution? No. Print resolution matters so that colors are accurate, solid areas are uniform, and gradients are smooth. It is also worth noting that digital printing far surpasses silver-halide printing in this respect.
Now, the image used for the print must contain enough information to withstand the enlargement of the chosen print. This is where things get a little more complicated.
Countless attempts can be found on the internet to explain the possible enlargement depending on the size (the number of pixels) of the image. It is generally accepted that a resolution of 240 dpi is sufficient for all prints, and 300 dpi even better.
It is indeed a simple rule, but one that has its limits.
Indeed, at 300 dpi, a 60 x 90 print is made up of 75 million pixels. How many commercially available cameras offer 75 million pixels today? None. Only very specific devices or stitched assemblies could reach that density.
The real question is: from what distance is this print likely to be viewed?
The human eye has a limited resolving power. Resolving power is the minimum angle needed between the eye and 2 points for those 2 points to be clearly distinguished rather than perceived as a single one. This angle is about 0,017° in humans. The minimum distance between two points for them to be discerned therefore depends on the distance between those points and the observer.
The following graph shows the eye's resolving power as a function of its distance from the object. The blue curve gives the distance in millimeters and the orange curve the corresponding resolution in dots per inch:
It can be verified that at a distance of 40 cm the human eye is no longer able to distinguish two points if they are less than 0.12 mm apart, which corresponds to a resolution of 214 dpi. At a distance of 2 m this value drops to 43 dpi. The rules of thumb stated above take quite a beating.
These are purely physiological and theoretical values. In practice, these values should therefore be adjusted. Indeed, while this graph tells us the resolution needed for a print viewed at a given distance, it does not tell us the typical viewing distance of a print according to its size.
To do this, we have developed an approximation formula. Explaining it would take too long and may be the subject of a future article, but it allows us to calculate the recommended resolution and the minimum advised resolution for a given print size.
The table for the most common print sizes is shown opposite.
As we can see, questions about the resolution of a print (or a printed output) are perfectly legitimate and certainly more complicated to answer than some would like to believe.
The information provided in this article challenges many preconceived ideas. In particular, it turns out that large-format printing is not the preserve of the lucky owners of high-end cameras. It all depends on how the print will be used. Of course, the more resolution you have available, the better.
Beyond 1m x 1m, you must prepare a file at (1/5) of the final size at 300 DPI. For larger sizes, we recommend using original images without software retouching or enlargement, with as many pixels as possible,(feel free to ask us for advice via the online chat).